The present invention relates to a method for operating a diesel engine.
Ever more stringent exhaust-emission regulations place increased demands on the operation of diesel engines, the intention being that appropriate measures for reducing the pollutant emission, in particular of soot, CO and NOx, should not lead to a reduction in the corresponding engine power and to an increase in the specific fuel consumption.
In conventional methods for operating diesel engines, the injection of fuel into a cylinder is divided into a pre-injection and a main injection. The pre-injection of a partial quantity of fuel in a period before the ignition upper dead center position can result in an improved homogenization of the fuel/air mixture. During the subsequent main injection, the ignition of the fuel/air mixture occurs in the region of the upper dead center position where at least a partial quantity of the mixture contributes to a reduction in the pollutant emission because of the improved homogenization. The main injection following the pre-injection may be drawn into the working cycle, in which case the pressure dropping in the cylinder during the working cycle may cause a reduction in the efficiency and along with that an increase in the fuel consumption.
It is an object of the present invention to provide a method for operating a diesel engine with reduced pollutant emission.
The above and other beneficial objects of the present invention are achieved by providing a method for operating a diesel engine as described herein.
For this purpose, the injection is performed in all operating states at a sufficiently late point that it begins after the ignition upper dead center position, the injector valve being opened particularly rapidly, i.e., abruptly. By the particularly rapid release of the cross section and the associated xe2x80x9csharpxe2x80x9d injection profile, a particularly good homogenization of the fuel/air mixture is obtained, and along with that a reduction in the formation of soot. This effect is assisted by a corresponding, high closing gradient of the injector valve, i.e., by an abrupt closing thereof. This xe2x80x9csharpxe2x80x9d injection profile may be further emphasized by an enlarged opening of the injector valve and, in particular, by an increased injection pressure, as a result of which a further improvement in the homogenization of the fuel/air mixture may be obtained. In conjunction with the late beginning of the injection in the working cycle after the upper dead center position, the distribution of fuel in the combustion chamber is assisted by a prolonged ignition delay, which is assisted by the decreasing cylinder pressure in the expansion phase. The homogenized mixture burns at an extremely high speed, with little soot being formed. Furthermore, a reduction in the nitrogen oxide emission and in the development of noise may be obtained thereby. The injection may be performed as a main injection which is preceded by pre-injection in the compression cycle. The opening of the injector valve, and also the closing thereof, occurs in the main injection abruptly and with a higher gradient than in the pre-injection. Through the abrupt opening and closing of the injector valve in the main injection, the foregoing advantages are obtained, while the ignition performance of the fuel/air mixture is additionally improved by the pre-injection.
The homogenization of the fuel/air mixture may be assisted by division of the main injection into a plurality of sub-injections. In conjunction with an exhaust-gas recirculation system, the nitrogen oxide emission which has in any case already been reduced may be further reduced. Even with increasing exhaust-gas recirculation, the described homogenization of the cylinder charge does not lead to a rise in the formation of soot.
An afterinjection may be performed following the main injection, which results in an increase in the combustion-chamber temperature and the HC content in the exhaust gas. In conjunction with an exhaust-gas recirculation system, the postoxidation of soot within the engine may thereby be improved.
A homogenizing injection which begins during the induction cycle may be performed prior to the pre-injection. The appropriate fuel quantity is injected at a low cylinder pressure, which promotes homogenization of the fuel/air mixture. In conjunction with the pre-injection, an improvement in the ignition conditions in the main combustion further results therefrom.
As a consequence of the improved homogenization of the fuel/air mixture, the method according to the present invention may provide a reduction in the pollutant emission without a loss in efficiency even if the main injection occurs at a late point.